The present invention generally relates to carbon nanotubes, and more specifically relates to a technique for precisely transferring a carbon nanotube pattern from a photomask to a wafer.
Carbon nanotube technology is fast becoming a technological area to make an impact in electronic devices. Single-wall carbon nanotubes (CNTs) are quasi-one dimensional nanowires, which exhibit either metallic of semiconducting properties, depending upon their chirality and radius. Single-wall nanotubes have been demonstrated as both semiconducting layers in thin film transistors as well as metallic interconnects between metal layers.
Currently, there are two approaches which are being used to pattern CNTs (i.e., to transfer a carbon nanotube pattern from a photomask to a wafer):
FIGS. 1-3 illustrate a first method which is currently used. In each one of FIGS. 1-3, a top view is provided on the left, and a side view is provided on the right. In the method, as shown in FIG. 1, initially a CNT layer 10 is provided on a substrate 12 and a resist 14 is patterned on the CNT layer 10. Then, as shown in FIG. 2, O2 plasma is used to etch the CNTs 10 (i.e., from the locations identified by reference numeral 16). Then, as shown in FIG. 3, the resist is stripped using wet chemistry.
Disadvantages of this method include the fact that the O2 plasma tends to lateral etch both the CNTs and resist. Where the resist lateral dimension reduces, the final CNTs pattern line width also decreases as indicated in the progression of FIG. 1 to FIG. 2, wherein the width of both the CNTs and the resist has decreased (despite the fact that the plasma etching was intended to merely etch any CNT which was not covered by the resist). Actually, the higher the pressure of the O2 plasma, the more dimension loss there tends to be. In addition, it is usually difficult to use wet strip chemistry to strip the organic antireflective layer (i.e., the resist). Therefore, resist patterning often remains on the (CNTs), and this may lead to high reflectance for the pattern light, and poor profile for the resist patterns. All this results in difficult control of the critical dimension of the CNT pattern.
FIGS. 4-6 illustrate a first method which is currently used. In each one of FIGS. 4-6, a top view is provided on the left, and a side view is provided on the right. In the method, as shown in FIG. 4, initially a CNT layer 10 is provided on a substrate 12, a hard mask layer 13 is provided on the CNT layer 10, and a resist 14 is patterned on the hard mask layer 13. Then, as shown in FIG. 5, the hard mask 13 and the CNT layer 10 is etched away (i.e., those portions which are not covered by the resist—identified with reference numeral 16 in FIG. 5). Finally, as shown in FIG. 6, the resist is ashed away using O2 plasma.
Disadvantages of this method include the fact that after the hard mask and CNT layers are etched, the O2 plasma which is used to ash the resist attacks the CNTs from the hard mask sidewall (i.e., the plasma undercuts the hard mask and attacks the CNTs underneath). This results in a resulting, physical CNT pattern which is narrower than that of the design.